[pim] Re: Erik Kline's Discuss on draft-ietf-pim-zeroconf- mcast-addr-alloc-ps-07: (with DISCUSS)

Erik,

That’s a great suggestion. We added the underlined to our -10 update to hopefully make this easier to understand:

An additional concern is that the group IDs used for this dynamic range overlap with the range used for Solicited-Node multicast addresses, a special type of multicast used by IPv6 for neighbor discovery (see Section 2.7.1 of RFC4291). This overlap increases the risk of unintentional conflicts with link-layer addresses.

Best Regards,

Nate

From: Erik Kline <[email protected]>
Sent: Thursday, November 20, 2025 09:40
To: Karstens, Nate <[email protected]>
Cc: The IESG <[email protected]>; [email protected]; [email protected]; [email protected]
Subject: Re: [pim] Erik Kline's Discuss on draft-ietf-pim-zeroconf-mcast-addr-alloc-ps-07: (with DISCUSS)

> Please provide an example of how multicast addresses with T=1 overlap with ff02: : 1: ffxx: xxxx addresses. Sure, I will base this example off of the example in RFC 4489 section 4. Let’s say that a device’s IPv6 link local address is fe80: : a12: 34ff: fe56: 7890. 


> Please provide an example of how multicast addresses with T=1 overlap with ff02::1:ffxx:xxxx addresses.

Sure, I will base this example off of the example in RFC 4489 section 4.

Let’s say that a device’s IPv6 link local address is fe80::a12:34ff:fe56:7890. It will use a Solicited-Node multicast address of ff02::1:ff56:7890. It uses RFC 4489 to calculate a link-scoped multicast prefix of ff32:00ff:a12:34ff:fe56:7890::/96. If it chooses 0xff567890 as its group ID (which is compliant with host allocation described in RFC 3307 section 4.3.2), then it will use the IPv6 multicast address ff32:00ff:a12:34ff:fe56:7890:ff56:7890. Both of these IPv6 multicast addresses use Ethernet multicast address 33:33:ff:56:78:90.

If you think this example would be useful to include in the document, how would you feel about updating draft-ietf-pim-updt-ipv6-dyn-mcast-addr-grp-id<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-ietf-pim-updt-ipv6-dyn-mcast-addr-grp-id/__;!!EJc4YC3iFmQ!Q9ih46cUb4H0_ynEnS6hu6LbP0DYub-DPyS_FqbGAKwXIxZlUuxJ4d9xJFvsrO1C375rub4lixse0xcsvA$> to include it? That document is focused specifically on improving the IPv6 multicast address architecture to better accommodate multicast address allocation protocols.

Thank you; it's overall more clear that the "collision" that is concerned here is (effectively) the resulting MAC address.

I think in this case what triggered me was "this dynamic range overlaps", which confused me as the IPv6 prefixes do not overlap.  I think if you just clarify that the group ID/MAC address is the thing that overlaps (or collides) it would be helpfully more accurate.

Thanks for bearing with me,
-Erik

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